Method for heat-treating work pieces made of temperature-resistant steels

ABSTRACT

A method of producing a workpiece of a heat-resistant steel, such as hot forming tool steel, where the workpiece may be hardened and depassivated after mechanical machining and electrochemical treatment. The hardening including a reduction step, so that no depassivation need be performed by pickling, for example, before nitriding. The result of the hardening treatment is a favorable surface condition for stepwise nitriding.

FIELD OF THE INVENTION

The present invention relates to a method for heat treatment of aworkpiece made of heat-resistant steel, in particular hot forming toolsteel, the workpiece being hardened and nitrided after mechanicalworking and electrochemical treatment, reduction of the workpiecesurface being performed during hardening without having to perform apickling treatment before the subsequent nitriding.

BACKGROUND INFORMATION

Nozzle bodies for modern direct injection systems are used to anincreasing extent at operating temperatures up to 450° C. High demandsare therefore made on the strength of components and the wear resistanceof nozzle bodies. Nitrided hot forming tool steel in particular istherefore used to manufacture the nozzle bodies. ECM (electrochemicalmachining) methods are used in the production of internal bores(pressure chambers) and for rounding. The ECM methods used for shapingand surface treatment of metal workpieces are performed in anelectrolyte solution, the workpiece to be machined usually beingconnected as the anode and the tool being connected as the cathode.Electrochemical machining methods are used in particular for deburring,polishing, grinding and etching the surfaces of a workpiece. Theworkpieces formed by the ECM method are highly passive and are verydifficult to treat by thermochemical diffusion methods, in particularnitriding, because more noble alloy elements such as Cr remain on thesurface and/or oxide alloy elements become oxidized, forming metaloxides and metal hydroxides Me_(x)O_(y)[OH]_(z).

To improve the nitridability of direct-injection nozzle bodies, it isconventional today to pickle passive surfaces before nitriding, inparticular by using hydrochloric acid. However, pickling has some majordisadvantages. Pickling with acid may cause pickling scars, whichdecrease the strength of the component. Furthermore, it is verydifficult to reproduce the results of pickling, because the length ofstorage between machining, basic heat treatment and nitriding may vary.Furthermore, pickling results in a considerable additional cost which isattributable in particular to the cost of the installation used forpickling and the required labor cost. Pickled workpieces must also becleaned after pickling by using a very complex special cleaningtechnique. Disposal of pickling solutions is also complicated. Inaddition, pickling with acid results in unwanted environmental pollutionand has a negative effect on working conditions.

The object of the present invention is thus to develop a method oftreating workpieces made of hot forming tool steel, in particular directinjection nozzle bodies, to improve the nitridability of theseworkpieces in particular without having to pickle the workpieces and tothus avoid the disadvantages due to pickling which are known in therelated art.

SUMMARY

The present invention is a method of producing a workpiece of aheat-resistant steel, in particular a hot forming tool steel, theworkpiece being hardened and thereby depassivated, characterized in thatthe hardening step includes a reduction treatment, in particular byusing hydrogen, and then according to the present invention, thetempered workpieces having the active surface are nitrided in severalsteps under different gas atmospheres, the nitriding being performedfirst in an atmosphere of ammonia and an oxidizing agent, in particularwater vapor or air, and then in an atmosphere of ammonia and acarbonaceous gas, in particular endogas or a mixture containing COand/or CO₂.

The advantages of the method according to the present invention for heattreatment and of heat-resistant workpieces produced in this way from hotforming tool steel, in particular direct-injection nozzle bodies, arethe result in particular of eliminating the pickling treatment beforenitriding. Since no pickling is performed according to the presentinvention, no pickling scars are formed on the surface of the workpiece.Therefore, workpieces produced in this way have very advantageousstrength properties. Since the method according to the present inventiongreatly improves the nitridability of the workpiece surfaces, theworkpieces are also characterized by extremely uniform entire internaland external nitride layers. The method according to the presentinvention is also much less expensive in comparison with the methodknown in the related art because the installations required for picklingand subsequent cleaning are eliminated, and only equipment for supplyinghydrogen to the vacuum hardening installation is needed. Since no acidsare used for pickling in the method according to the present invention,this definitely results in less environmental pollution, and inparticular it also improves working conditions.

DETAILED DESCRIPTION

A workpiece made of a heat-resistant steel, such as a hot forming toolsteel, may be hardened and thereby depassivated, and the hardening stepmay include a reduction treatment. This reduction may cause metal oxidelayers and/or metal hydroxide layers on the surface of the workpiece tobe removed, so that the subsequent nitriding may be greatly improvedwithout having to perform pickling. The reduction treatment may beperformed by using hydrogen.

In conjunction with the present invention, a hot forming tool steel isunderstood to be a steel which is constantly exposed to an elevatedtemperature during its use, in particular a temperature of more than200° C. There must not be any structural changes in hot forming toolsteel during use, but instead the structure must be sufficiently stableand must have good tempering properties. Hot forming tool steel musthave different properties depending on the desired application.Important desired properties include in particular strength andhardness, which in turn determine wear resistance.

Hot forming tool steel must meet some special requirements with regardto use properties, including hot strength, which is achieved inparticular by molybdenum, tungsten and fine-grained vanadium, goodtempering properties, which are achieved by chromium, which togetherwith molybdenum, nickel and manganese increases hardenability, and hotwear resistance, which may be determined by the heat strength of thematrix and by the type and amount of special carbides. Direct-injectionnozzle bodies of hot forming tool steel must have a very high wearresistance, for example.

In one exemplary embodiment of the present invention, the workpiece madeof a heat-resistant steel, in particular hot forming tool steel, may bemechanically machined and subjected to an electrochemical machiningbefore hardening, i.e., to an ECM method which is performed in anelectrolyte solution for shaping and surface treatment. Such a methodmay be used in particular for deburring, polishing, grinding and/oretching the workpiece. For example, internal bores may be produced byusing an ECM method and rounding subsequently.

The workpiece may be subjected to cleaning in an aqueous cleaningmedium, in particular a neutral cleaning agent, after the ECM method.The cleaning step may prevent the development of thick layers ofMe_(x)O_(y)[OH]_(z) on the surface of the workpiece. Following thecleaning step, the workpiece may be dried. Next the workpiece may behardened immediately. In one embodiment of the present invention, theworkpiece may be first preserved by suitable methods if it is to bestored for a prolonged period of time after the ECM machine; then afterstorage, immediately before hardening, it may be cleaned again in aliquid cleaning medium.

Hardening which results in a change in structure of the hot forming toolsteel as described above may be performed in a single-chamber ormultichamber vacuum furnace. Hardening may include convective heating ofthe workpiece under nitrogen. Convective heating of the workpiece may beperformed under a nitrogen pressure greater than 0.8 bar. In anotherembodiment of the present invention, the workpiece may also be heated invacuo. The workpiece may be heated at least up to the hardeningtemperature of the hot forming tool steel. The hardening temperature ofhot forming tool steel may be approximately 1040° C.

After reaching a desired temperature, the nitrogen atmosphere or thevacuum may be replaced by hydrogen. The hydrogen thus introduced may actas a reducing agent for reduction of the layers of metal oxide and/ormetal hydroxide present on the tool surface and may be introduced at atemperature of at least 400° C. However, the temperatures at whichhydrogen is introduced may be in the range of the hardening temperature.The hydrogen partial pressure may be approximately 1 to 100 mbar. Theflow rate of the hydrogen feed may be 100 to 2000 L/h. Austenitizationmay be performed over a period of 10 to 40 minutes.

In another embodiment of the present invention, the gas exchange may beperformed as a pulsating operation over a period of one to ten minutes.In other words, the hydrogen partial pressure may be increased in apulsating manner over a period of one to ten minutes in exchange withvacuum. This yields a better gas exchange, in particular with workpieceshaving blind boreholes.

The hydrogen may be pumped out before the end of austenitization toprevent the gas used for quenching in the following step from becomingcontaminated with hydrogen.

The austenitized workpiece may be quenched in nitrogen at a pressure of1 to 10 bar after holding it at the hardening temperature.

After hardening, in particular after quenching, the workpiece may besubjected to at least one tempering step.

The workpiece may be tempered at a temperature of up to 650° C., thetempering of the workpieces taking place either in a nitrogen atmosphereor under a nitrogen-hydrogen atmosphere. When a nitrogen-hydrogenatmosphere is used, it may contain up to 5% hydrogen. Tempering of theworkpiece may be performed in a vacuum furnace or an evacuable temperingfurnace. The tempering step may be performed for approximately one totwo hours.

There is the possibility of the workpiece being subjected to multipletempering steps instead of just one. In one embodiment, the workpiecemay be subjected to a first tempering step which lasts approximately oneto two hours, during which it is heated to a temperature of 520° C., andfollowing that it may be subjected to a second tempering step, which maylast approximately one to two hours and during which it may be heated toa temperature of 610° C.

The workpiece may be nitrided after tempering. Nitriding results inhardening of the hot forming tool steel of which the workpiece is made.This is based on diffusion of nitrogen into the steel. This results inan incorporation of nitrogen at interlattice sites and formation ofnitrides and addition of nitrogen onto carbides to form carbonitrides.Nitriding results in hard boundary areas, thus increasing the hardness,wear resistance and durability of the hot forming tool steel.

The workpiece may be transferred to a nitriding furnace immediatelyafter hardening and tempering. The nitriding furnace used may be apurged chamber furnace or an evacuable retort oven.

In one embodiment of the present invention, the workpieces in thenitriding furnace may be heated from room temperature to a temperatureof approximately 400° C. in a first step. Heating of the workpieces inthe nitriding furnace may be performed in an ammonia atmosphere. Then ina second step the workpiece may be heated up to the nitridingtemperature, which is approximately between 500° C. and 600° C.Nitriding of the workpieces, which is performed following heating, mayinclude the following steps:

-   step 1: nitriding in an atmosphere of ammonia and an oxidizing    agent,-   step 2: nitriding in an atmosphere of ammonia and a carbonaceous    substance,-   step 3: nitriding in an atmosphere of ammonia or a gas additive to    reduce the nitriding index.

In other words, the workpiece may be nitrided in a gas atmosphere whichmay be changed incrementally. The oxidizing agent in step 1 may be 0.5to 10 vol % water vapor or up to 15% air. The carbonaceous substanceused in step 2 may be 1 to 10 vol % endogas. Endogas is obtained byendothermic reaction of hydrocarbons such as propane and is a mixture of23.7 vol % CO, 31.5 vol % H₂ and 44.8 vol % N₂. In another preferredembodiment, CO and/or CO₂ may also be used in equivalent amounts as thecarbonaceous substance. The nitriding in step 2 is referred to as gasoxycarburation and may last more than four hours or betweenapproximately 10 to 60 hours. After the gas oxycarburation reaction,which may last more than four hours, a uniform nitride layer has alreadydeveloped on the surface of the workpiece. Following step 2, i.e., instep 3, a treatment may be performed in ammonia or by adding gas toreduce the nitriding index in order to reduce the growth of connectinglayers.

The gas flow rate during nitriding depends on the effective furnacevolume and may amount to three times the effective furnace volume inL/h.

The workpieces may be cooled by using nitrogen after nitriding. Theworkpiece produced and treated by using the method according to thepresent invention may then be hard machined by conventional methods.

The method according to the present invention may be used to produceheat-resistant direct-injection nozzle bodies of hot forming tool steel,the nozzle body being made of high-strength heat-resistant hot formingtool steel, such as steel brands X40CrMoV51 and X38CrMoV51. The pressurechamber may be machined further, and a manufacturing cycle whichincludes soft machining, ECM machining and subsequent directly linkedcleaning in an aqueous cleaning medium, but no pickling treatment, isperformed according to the present invention. Then the direct-injectionnozzle bodies may be hardened in a vacuum furnace in the temperaturerange between 1000° C. and 1070° C. under a pulsed hydrogen partialpressure of 1 to 100 mbar and next quenched in a stream of nitrogen gasat a pressure of 1 to 10 bar. Tempering may be performed at atemperature of up to 650° C. in a nitrogen atmosphere or anitrogen-hydrogen atmosphere. Subsequent nitriding may be performed at510° C. to 590° C. over a period of 10 to 60 hours using the gasoxynitrocarburation method described above in a chamber furnace or anevacuable chamber furnace. Heat-resistant direct-injection nozzlesbodies treated in this way have more advantageous strength propertiesbecause the nitride layer is uniformly developed and there are nopickling scars like those described in the related art.

1. A method for producing a workpiece from a heat-resistant steel,comprising the steps of: hardening the workpiece, including a reductiontreatment to form a depassivated surface for stepwise nitriding, andconvective heating of the workpiece under one of a nitrogen atmosphereand in vacuo; nitriding the workpiece; replacing the one of the nitrogenatmosphere and the vacuum by a hydrogen atmosphere after reaching apredetermined heating temperature; and generating the hydrogenatmosphere in a pulsating operation over a pulse period of between aboutone and ten minutes.
 2. The method as claimed in claim 1, whereinhydrogen is used as a reducing agent.
 3. The method as claimed in claim1, further comprising machining and electrochemically treating theworkpiece before the hardening step.
 4. The method as claimed in claim1, further comprising cleaning the workpiece before the hardening step.5. The method as claimed in claim 4, further comprising drying theworkpiece after the cleaning.
 6. The method as claimed in claim 1,further comprising cleaning the workpiece in an aqueous cleaning mediumbefore the hardening step.
 7. The method as claimed in claim 1, whereinthe hardening step includes convective heating of the workpiece under anitrogen atmosphere with a nitrogen pressure greater than 0.8 bar. 8.The method as claimed in claim 1, wherein the heat-resistant steelincludes a hot forming tool steel and wherein the workpiece is heated atleast to a hardening temperature of the hot forming tool steel.
 9. Themethod as claimed in claim 1, wherein a hydrogen partial pressure isbetween about 1 and 100 mbar.
 10. The method as claimed in claim 1,wherein a hydrogen flow rate is between about 100 and 2000 L/h.
 11. Themethod as claimed in claim 1, wherein the hardening step is performed inone of a single-chamber and multichamber vacuum furnace.
 12. The methodas claimed in claim 1, further comprising quenching the workpiece afterthe hardening.
 13. The method as claimed in claim 12, wherein theworkpiece is quenched in the quenching step using nitrogen.
 14. Themethod as claimed in claim 12, wherein the nitrogen has a pressure ofbetween about 1 and 10 bar.
 15. The method as claimed in claim 1,further comprising tempering after the hardening.
 16. The method asrecited in claim 15, wherein the tempering step includes heating theworkpiece up to a temperature of about 650° C.
 17. The method as claimedin claim 15, further comprising heating the workpiece in a nitrogenatmosphere.
 18. The method as recited in claim 15, further comprisingheating the workpiece in a nitrogen-hydrogen atmosphere having ahydrogen content of up to about 5%.
 19. The method as claimed in claim15, wherein the tempering is performed in one of a vacuum furnace and anevacuable tempering furnace.
 20. The method as claimed in claim 15,wherein the tempering is performed over a period of between about 1 and4 hours.
 21. The method as claimed in claim 1, further comprising in afirst step heating the workpiece from room temperature up to atemperature of approximately 400° C.
 22. The method as claimed in claim21, wherein the workpiece is heated in the heating step under an ammoniaatmosphere.
 23. The method as claimed in claim 1, wherein the workpieceis heated up in the heating step to a nitriding temperature.
 24. Themethod as claimed in claim 1, wherein the nitriding of the workpieceincludes the steps of: (a) nitriding under an atmosphere of ammonia andan oxidizing agent; (b) nitriding under an atmosphere of ammonia and acarbonaceous substance; and (c) nitriding under an atmosphere of one ofammonia and a gas additive.
 25. The method as claimed in claim 24,wherein the carbonaceous substance includes one of about 1 to 10 vol %endogas and CO and CO₂ in equal amounts.
 26. The method as claimed inclaim 24, wherein the oxidizing agent including one of about 0.5 to 10vol % water vapor and up to 15% air.
 27. The method as claimed in claim1, further comprising cooling the workpiece under nitrogen after thenitriding.
 28. The method as claimed in claim 1, further comprising hardmachining the workpiece after cooling.
 29. The method as claimed inclaim 1, wherein the workpiece includes a direct-injection nozzle body.30. The method as claimed in claim 1, wherein the heat-resistant steelincludes hot forming tool steel.